With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted.
In the initial WLAN technology, a frequency of 2.4 GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared communication, etc. Recently, the WLAN technology can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplex (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to above 540 Mbps, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate. In addition, this standard may use a coding scheme which transmits several duplicate copies to increase data reliability and also may use the OFDM to support a higher data rate.
An IEEE 802.11n HT WLAN system employs an HT green field physical layer convergence procedure (PLCP) protocol data unit (PPDU) format which is a PPDU format designed effectively for an HT station (STA) and which can be used in a system consisting of only HT STAs supporting IEEE 802.11n in addition to a PPDU format supporting a legacy STA. In addition, an HT-mixed PPDU format which is a PPDU format defined such that a system in which the legacy STA and the HT STA coexist can support an HT system.
With the widespread use of the WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput in comparison with a data processing rate supported by the IEEE 802.11n. A very high throughput (VHT) WLAN system is a next version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing rate of above 1 Gbps in a medium access control (MAC) service access point (SAP).
The VHT WLAN system allows a plurality of VHT STAB to simultaneously access a channel in order to effective use a radio channel. For this, multi-user multiple input multiple output (MU-MIMO)-based transmission using multiple antennas is supported. A VHT access point (AP) can perform spatial division multiple access (SDMA) transmission for transmitting spatial-multiplexed data to the plurality of VHT STAs. When data is simultaneously transmitted by distributing a plurality of spatial streams to the plurality of STAs by the use of a plurality of antennas, an overall throughput of the WLAN system can be increased.
In a multi-user environment in which one AP supports a plurality of STAs, there is on-going research on multiple transmission and reception antenna transmission techniques or the like by considering multiple users in order to increase overall channel capacity of a MU-MIMO system considering multiple users. The multi-user channel environment needs to guarantee that a channel matrix is in a good state so that all MU-MIMO schemes can fully use spatial flexibility. Thus, it is required that the multiple users can simultaneously perform transmission with respective desired data transfer rates without being restricted by interference. Since the AP transmits a radio signal simultaneously to several STAs in a downlink channel, each STA receives a signal of another user in addition to a desired signal, which may act as interference. To suppress the interference, the AP can perform channel filtering to cancel the interference. For example, a zero-forcing filter can be used to decrease the interference.
The VHT WLAN system can support a wider bandwidth than an HT WLAN system. However, similarly to the HT WLAN system, not all STAs can support the greatest bandwidth supported by the system. When two STAs support different bandwidths, if simultaneous channel access is disabled, the STAs access and use the channel in a time division manner, which results in deterioration of efficiency in the use of radio resources. Therefore, if the AP can receive data simultaneously from a plurality of STAs which use different bandwidths as a transmission channel bandwidth, an overall throughput of a WLAN system consisting of the AP and the plurality of STAs can be more improved. For this, there is a need for a method which allows the AP to be able to simultaneously receive data from the plurality of STAs.